1. Field of the Invention
The present invention relates generally to apparatus and methods for characterizing physical properties of an eye and more specifically for characterizing cataracts.
2. Description of the Related Art
The term cataract refers to a reduction in transparency of the lens within the eye. Although some cataracts develop rapidly, the vast majority develop over a period of decades, becoming increasingly prevalent in older populations. Surgery to remove cataractous lenses is currently a very common operation, performed by ophthalmologists, usually as an office procedure. This procedure typically includes the replacement of the cataractous lens with a plastic one, called an intraocular lens (IOL).
For virtually all cataracts, the physical process that reduces the transparency of the lens is not one that causes light absorption and thus opacity. Rather, the transparency is reduced because the material that forms the lens changes its structure in such a way that the lens scatters light. This light scatter impairs vision because it adds a veiling glare in the retinal image, reducing the contrast of the scene viewed.
Cataract surgery is not entirely without risk, and because cataracts usually develop so slowly, the question of when it is appropriate to perform the surgery to remove them is sometimes difficult. Surprisingly, as of this writing there is no widely accepted method, either subjective or objective, for determining when surgery is appropriate. One might expect that measures of visual acuity would be appropriate, but they have been shown not to be useful for making the decision about whether or not to undergo surgery. Methods currently used to evaluate cataracts for research on their prevention utilize a mixture of objective and subjective methods, namely, photographs taken using a standard ophthalmic instrument, the “slit lamp”, are visually compared with a set of standardized photographs to pick which standard photograph looks most like the patient's.
Cataracts have long been observed by an optical method called retroillumination. With this method, the retina is illuminated, usually using a slit lamp, and the light reflected from the retina back-illuminates the lens in such a way that regions containing cataractous material appear darker than clear areas.